[x/go.dev] content/about.md: add information about custom links

Refer to the issue that describes how to create custom links
that appear in the right sidebar.

Change-Id: I75775bb99f9ab44f51c3802541b92d4835301c8e
X-GoDev-Commit: 28e1cdcc70d609ee3f8e31dd7a31a9e333c604e8
diff --git a/go.dev/content/about.md b/go.dev/content/about.md
index d0f1556..312750e 100644
--- a/go.dev/content/about.md
+++ b/go.dev/content/about.md
@@ -45,6 +45,12 @@
 
 [![PkgGoDev](https://pkg.go.dev/badge/golang.org/x/pkgsite)](https://pkg.go.dev/golang.org/x/pkgsite)
 
+## Adding links
+
+You can add links to your README files and package documentation that will be
+shown on the right side of the pkg.go.dev page. For details, see [this
+issue](https://golang.org/issue/42968).
+
 ## Keyboard Shortcuts
 
 There are keyboard shortcuts for navigating package documentation pages. Type '?' on a package page for help.